Thank you for your help!
I was finally able to get a WPF/XAML UserControl GUI to appear inside of a Win32 HWND.
The WPF .dll needed to be signed and added to the Global Address Cache… since I haven’t created an installer yet, I used the Visual Studio command prompt and the gacutil command.
project structure:
1.Plugin Processor/EditController/CPluginView C++ .dll
2.C++/CLI wrapper/trampoline/middle-man .dll
3.C#/XAML UserControl .dll